Problem: Simplify the following expression: $y = \dfrac{2x^2+17x+36}{x + 4}$
Explanation: First use factoring by grouping to factor the expression in the numerator. This expression is in the form ${A}x^2 + {B}x + {C}$ First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(2)}{(36)} &=& 72 \\ {a} + {b} &=& &=& {17} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $72$ and add them together. The factors that add up to ${17}$ will be your ${a}$ and ${b}$ When ${a}$ is ${9}$ and ${b}$ is ${8}$ $ \begin{eqnarray} {ab} &=& ({9})({8}) &=& 72 \\ {a} + {b} &=& {9} + {8} &=& 17 \end{eqnarray} $ Next, rewrite the expression as $({A}x^2 + {a}x) + ({b}x + {C})$ $ ({2}x^2 +{9}x) + ({8}x +{36}) $ Factor out the common factors: $ x(2x + 9) + 4(2x + 9)$ Now factor out $(2x + 9)$ $ (2x + 9)(x + 4)$ The original expression can therefore be written: $ \dfrac{(2x + 9)(x + 4)}{x + 4}$ We are dividing by $x + 4$ , so $x + 4 \neq 0$ Therefore, $x \neq -4$ This leaves us with $2x + 9; x \neq -4$.